Apple prevents computers with USB drives from installing Windows from a USB flash drive on certain models. While it works flawlessly on my current gen MacBook Air, installing Windows 7 off a USB key doesn’t work on my iMac.

Luckily, the fix is easy…

From the System Information app (located in the Utilities folder on your hard drive), locate the Model Identifier. My Mac says “iMac12,1”.

Right click on Bootcamp, select Show Package Contents. Open Contents. Edit Info.plist in a text editor like Text Wranger or BBEdit.

Find this line:

<key>PreUSBBootSupportedModels</key>

I notice that in the entries beneath this line there is a reference to iMac12,2. This means that Apple is restricting the use of USB to models *after* iMac12,2.

My confirmed fix is to edit that line to read:

<string>iMac11,1</string>

so that my iMac12,2 can use USB.

Voila, it works!